1. Field of the Invention
The present invention relates to RF power amplifiers and amplification methods. More particularly, the present invention relates to feed forward amplifiers and methods for controlling feed forward amplifiers.
2. Description of the Prior Art and Related Background Information
RF amplifiers are devices that attempt to replicate an RF signal present at an input, producing an output signal with a much higher power level. The increase in power from the input to output is referred to as the ‘gain’ of the amplifier. When the gain is constant across the dynamic range of the input signal, the amplifier is said to be ‘linear’. Amplifiers have limited capacity in terms of power delivered because of gain and phase variances, particularly saturation at high power, which makes all practical amplifiers nonlinear when the input power level varies. The ratio of the distortion power generated relative to the signal power delivered is a measure of the non-linearity of the amplifier.
In RF communication systems, the maximum allowable non-linearity of the amplifier is specified by government agencies such as the FCC or the ITU. Because amplifiers are inherently nonlinear when operating near saturation, the linearity requirements often become the limitation on rated power delivering capability. In general, when operating near saturation, the linearity of the amplifier degrades rapidly because the incremental signal power delivered by an amplifier is proportionally less than the incremental distortion power generated.
Various compensation approaches are conventionally applied to reduce the distortion at the output of the system, which in turn increases the rated power delivering capability. The preferred approach is feed forward compensation. In feed forward RF power amplifiers an error amplifier is employed to amplify main amplifier distortion components which are then combined out of phase with the main amplifier output to cancel the main amplifier distortion component. In general, feed forward compensation provides the power capability of the main amplifier and the linearity of the error amplifier.
Feed forward linearization of an amplifier is based on the matching of the gain and phase of parallel RF paths to either cancel the carrier (input) signal (loop 1) or to cancel the distortion (loop 2). The carrier cancellation is usually referred to as the ‘loop 1 error’, which is an estimate of the distortion of the main amplifier path. The distortion cancellation occurs within loop 2, and uses the loop 1 error to cancel the distortion of the main amplifier. The matching of the gain and phase in the respective loops is referred to as ‘loop alignment control’. When the alignment of loop 2 is correct, the distortion at the output is minimized, making the entire feed forward system more linear than the main amplifier alone. When the alignment of loop 1 is correct, the power through the error amplifier (which amplifies the loop 1 error) is limited. In most cases, the loop 1 alignment must be completed before the error amplifier of loop 2 is enabled. This ensures that the error amplifier is not over-driven, a condition that could produce unwanted distortion or device damage.
Most end users of feed forward power amplifiers have specifications limiting the time that the adaptive portion of the feed forward compensation can take to align the loops. As a result, it is important to have good initial alignments when the adaptive controller begins its search for the best (or sufficient) alignment. Some such specifications have times as low as 10 seconds.
In addition, it is important to have good alignment of loop 1 to limit the power entering the error amplifier. However, the loop 1 error power is roughly the product of the input power and the alignment quality (amount of carrier cancellation). As a result, the alignment quality of loop 1 may be modest if the input power is low. If the input power increases abruptly, the loop 1 error power will increase proportionally, potentially over-driving the error amplifier. Although  loop 1 will automatically adjust its alignment setting in response to the increase, fast changes in the alignment of loop 1 are preferred because it reduces the transient effect of over-driving the error amplifier.
There have been numerous prior approaches to feed forward linearization, the earliest dating to the 1920's. In earlier approaches, the alignment settings were static, with fixed settings for gain and phase, optimized for nominal operating conditions. Later approaches introduced look-up-tables for compensation of temperature and DC supply variations. Still later, adaptive methods were applied where the misalignments of the loops were measured internally and used for subsequent alignment adjustments. The loop 1 error power was typically used as a measure of the loop 1 misalignment. For measuring loop 2, a pilot signal was typically introduced within loop 1 to act as a ‘known distortion’. The detected pilot power at the output of loop 2 measured the misalignment of loop 2. Pilotless methods for measuring the misalignment of loop 2 have been implemented which are based on distortion measurements. In such systems the second loop convergence has significant dependence on the input signal and the distortion created.
Look-up tables have been used in both static and adaptive versions of the feed forward amplifier. They are used, typically, to compensate for temperature or DC supply variations. However, even when look-up tables are used in an adaptive feed forward system, they are used typically to control parts of the system that are not adaptive, such as the front-end voltage-controlled attenuator and phase shifter (which maintain the overall gain or phase of the system). That is, the loop adaptation and the look-up tables are decoupled.
As indicated above, look-up tables in the past have used a fixed structure. The input, such as temperature, is an index to an array. The indices are equally spaced across the range in ascending order, and corresponding alignment settings are stored within the array. This structure is well suited to memory chips because the index is equivalent to an address and the alignment setting is equivalent to the data. However, the look-up tables are usually based on experimental data (calibration) requiring significant time to fill-in the elements of the table. In addition, drift from component aging can make any look-up table obsolete, necessitating a re-calibration.
Another difficulty with look-up tables is that it can be extremely difficult to manage multi-dimensional arrays, which would be required if many operating conditions are present which affect the alignment quality. One can imagine the number of elements present in an equally spaced four-dimensional array. For example, 10 samples per dimension produces 10000 elements.
One technique of managing multiple indexing dimensions is to assume that the effects are separable. Separable conditions would allow the use of individual arrays for each operating condition, and the composite effect would be the sum of the individual adjustments. (Not unlike a Taylor series expansion where the partial derivatives are specified). Unfortunately, this approach is valid only for small (differential) alignment adjustments because any cross-correlation between dimensions is ignored. The largest error would occur at the corners of the multi-dimensional array. For example, a troublesome corner in the temperature, DC supply index space would be high temperature and low voltage. It is these corner locations that are tested, typically, by sophisticated customers for determining if the amplifier is compliant with specifications.
A related problem with the array-based look-up table is the selection of the sampling interval (separation between adjacent indices) within the index space. In general, the sensitivities of the gain and phase settings vary over the index space. The sampling density must be selected based on the most sensitive region of the index space. The remaining regions will be over-sampled. This problem of over-sampling is made more significant for multi-dimensional arrays.
There have been attempts to make look-up tables self-calibrating or self-generating. However, the fixed array structure is difficult to manage. The key problem encountered is ‘update fragmentation’. Consider the previously-mentioned four-dimensional array case. When the look-up table is updated, only one element of 10000 is changed. If the source of the degradation is global (due to component drift, for example), then all 10000 elements are affected. However, the changes must propagate as each index is visited. The potential for neighboring indices to have large differences exists, simply because one of the indices is older.
Accordingly, a need presently exists for a system and method for rapid loop alignment control in a feed forward amplifier system which avoids the above noted limitations of the prior art.